A study of the remains of almost 20,000 people dating from the 8th century BC to the 18th century AD has found that the Roman empire reduced our level of nutrition, which increased again in the "dark ages"Jean M comments:
As the empire expanded, Roman soldiers found themselves fighting milk-fed Celts and Germans who towered over them. Diodorus Siculus described the Gauls as tall of body, with rippling muscles. Strabo marvels that mere lads from milk-drinking Britain were as much as half a foot taller than the tallest people in Rome. Tacitus declared that the Germanic tribes had huge frames. [. . .]
Nikola Koepke, "Nutritional status in pre-historic and historic Europe" was read at the The Economic History Society Annual Conference, University of Durham, 26-28 March 2010. It is online in the accompanying volume [pdf], pp. 13-18.